This invention relates generally to optical absorption spectroscopy, and more specifically relates to spectrophotometers operating in a dual beam mode.
In optical absorption spectroscopy, it is common practice to operate in a dual beam mode, and several well-known techniques may be utilized to achieve such operation. In one such method, e.g., the optical-wavelength energy exiting the instrument monochromator, is processed by a beam splitter, which directs approximately one-half of the light down a "sample" path, and simultaneously directs approximately the other half of the light down the second, or "reference" path. The samples to be studied, which may comprise gas, liquid or a solid, are placed in one or boths beams, in such a manner that the incident energy is either transmitted, reflected, or scattered by the sample. The beams are then caused to fall on two separate photodetectors, one for the sample beam, and one for the reference beam.
In a second method of dual beam operation, the light exiting the monochromator is directed alternately down the sample and reference path, by means of moveable and stationary optical elements. After contacting the sample, the beams are caused to fall on a single photodetector.
In a third method of dual beam operation, the sample and reference materials are moved alternately into a stationary beam which exits the monochromator. The modified beam is then caused to fall on a single photodetector.
In all of the above methods the light can be measured continuously by the photodetector, or the light can be "chopped." By the latter is meant that the beam may be periodically interrupted by an opaque or semi-opaque mask. In the second and third methods mentioned, the photodetector and the source of light are obviously interchangeable.
Regardless of which of the several above methods are employed, the object of dual beam spectroscopy is to produce two separate electrical signals, one of which is proportional to the intensity (I.sub.o) of the reference beam, and the other of which is proportional to the intensity (I) of the sample beam. It is also common practice to combine these two electrical signals, in such manner that either or both of the following outputs are produced: ##EQU1##
In utilizing dual beam spectrophotometers it is desirable to obtain an output of 100 percent transmission, or zero absorbance (corresponding to equal signals in each electrical channel) under various conditions, such as when there is no sample in either beam, when identical samples are in each beam, or under other conditions as may be determined by the user; and at all wavelengths at which the apparatus can be used. Since differences usually exist between the two channels, due to unavoidable optical or electrical differences in the beam paths or signal processing channels, it is necessary to amplify or attenuate the signal in one channel with respect to the other in order to achieve balance. In the past this has commonly been effected by the use of an electrical potentiometer which is coupled to the wavelength adjustment mechanism of the monochromator. The potentiometer has taps at various wavelengths, and these are connected to other manually-adjusted potentiometers. By careful adjustment of these potentiometers, sometimes called "multipots," it is possible to achieve a nearly "flat baseline," or zero absorbance level, throughout the entire range of the monochromator.
Various systems have furthermore, from time to time been proposed, the objective of which is to automate the baseline compensation scheme above mentioned, so as to eliminate the laborious and time-consuming operations inherent in multipot adjustment. In U.S. Pat. No. 3,646,331, for example, the combined reference and sample signals are digitized at selected discrete wavelengths, and a correction factor is calculated for each said discrete wavelength so that the digitized output multiplied by the correction factor will yield a zero absorbance output at each said wavelength. The correction factors are each stored, and when a sample is measured by the instrument, each of the stored factors are synchronously applied to multiply the signal derived from the sample. Similarly, U.S. Pat. No. 3,734,621 sets forth a concept wherein the signal indicative of the transmission characteristics of the spectrophotometer is stored during an initial or calibration run, for use in modifying the signals produced during a sample run, as to compensate for the instrument characteristics.
Notwithstanding the attention that has thus been directed toward automating baseline compensation so as to eliminate the requirement for manual adjustment of multipots, the prior art techniques have, in general, been based upon very complex technology. The net effect of these prior approaches has thus been of introducing high cost and reliability problems into the associated instruments.
In accordance with the foregoing, it may be regarded as an object of the present invention, to provide method and apparatus for dual beam spectroscopy, which automatically enables accurate and dependable baseline compensation, thereby eliminating the need for laborious and time-consuming manual adjustment.
It is a further object of the present invention, to provide method and apparatus for dual beam spectroscopy, which eliminates the need for manual baseline adjustment, and which automatically effects such adjustment without the use of excessively costly or complex techniques or components.